To reduce the amount of information of an image, compression coding methods such as JPEG, MPEG-1/2/4, and H.261/263/264 which utilize block-by-block transform are widely used. When an original image obtained by decoding a compression-coded still image or moving image frames, artificial noise, such as block noise and ringing noise, is generated in many cases. If a simple smoothing filter is applied to such artificial noise, patterns (edges and texture) inherent to the image may be lost though noise is eliminated.
In view of the above, a method has been proposed which can eliminate artificial noise effectively while maintaining patterns inherent to the image by using coding information (e.g., a block size as a unit of transform and transform coefficients and quantization information of the transform) that was used at the time of compression coding. An example of such method is disclosed in the following document.
Y. Yang et al. , “Regularized Reconstruction to Reduce Blocking Artifacts of Block Discrete Cosine Transform Compressed Images,” IEEE Trans. Circuits and System for Video Technology, Vol. 3, No. 6, December 1993.
The above conventional technique cannot be utilized for eliminating artificial noise effectively from an image from which coding information cannot be obtained.